1. Field of the Invention
This invention relates to a synthesizer of multifrequency code signals for a keyboard telephone station, more particularly to a generator producing dialling signals or more generally signalling signals embodied by frequencies associated m by m in a "m out of M" code, usually two by two in a "two out of M" code to represent the decimal dialling digits and, if necessary, other signalling-service signals. Such a dialling code can be e.g. the multifrequency code recommended by the CCITT (International Telegraph and Telephone Consultative Committee). In the multifrequency code used here the frequencies of the code are voiceband frequencies which, allowing for a tolerance, are integral sub-multiples of a single fundamental frequency.
2. Description of the Prior Art
Synthesizers of the kind considered are known, inter alia from patent application Ser. No. 617,488 filed Sept. 29, 1975 corresponding to French Pat. Application 74-32908, filed Sept. 30, 1974 in the names of Bernard Roche, Jacques Majos and Jean-Louis Lardy, which use a read only memory containing digital samples of a sine function corresponding to equidistributed values of the angular argument. The depression of a key of the keyboard selects two indices or increments i and ' and causes the read-out at a constant rate of the samples which are contained in the read only memory and which are read from i to i to produce a digital sinusoidal signal at a first frequency and from i' to i' to produce a digital sinusoidal signal at a second frequency.
Although the invention of the copending U.S. application Ser. No. 617,488 has due regard for the symmetry of the sinusoidal function so as to store only digital samples of the function corresponding to equidistributed values of the angular argument only lying between 0 and .pi./2, the need to have a store for sinusoidal-function digital samples in the dialling generator of each keyboard telephone stations adds considerably to the cost of such generator, and it is an object of this invention to make it unnecessary to use such a store.